The present invention relates to a process for producing a semiconductor device, more particularly to a process for producing a bipolar type semiconductor device. In prior art integrated circuits (IC) of bipolar type semiconductor devices, integrated injection logic (I.sup.2 L) and other circuits which are operated under a low tolerance voltage and at a high operating speed, and linear IC's and other circuits which can be operated under a high tolerance voltage, are often provided on the same chip. In such a case, as shown in FIG. 1, it is preferable that in the I.sup.2 L element region 1, the distance d.sub.1 between P type regions 6, 7 and 8 and the N.sup.+ type buried region N.sup.+ b.sub.1 be as small as possible so as to obtain a high operation speed. It is also preferble that in the linear element region 2, the distance d.sub.2, between the P type region 9 and the N.sup.+ type buried region N.sup.+ b.sub.2, be somewhat large so as to obtain a high tolerance voltage. In the FIG. 1, reference numerals 4 and 5 show a P type semiconductor substrate and N.sup.+ region, respectively.
In forming a bipolar type semiconductor device having the above-mentioned conventional structure, as shown in FIG. 2a, N type impurities having different diffusion coefficients, such as arsenic (As) and phosphorus (P), are introduced into the I.sup.2 L element region 1 and linear IC element region 2 to form the N.sup.+ type diffusion regions N.sup.+.sub.AS and N.sup.+.sub.P within the P type semiconductor substrate 4.
Then, as shown in FIG. 2b, as shown in FIG. 2b, an N type epitaxial layer 3 having a required thickness is formed on the P type semiconductor substrate 4 by chemical vapour deposition at a temperature of 1000.degree. C. to 1200.degree. C. Since the diffusion coefficient of arsenic is different from that of phosphorus, the amount of arsenic diffused in the N.sup.+.sub.AS type region differs from that of phosphorus. Thus, the N.sup.+ buried regions N.sup.+ b.sub.1 and N.sup.+ b.sub.2, are formed, with a different thickness between the P type epitaxial layer 4 and N type epitaxial layer 3. Thickness t.sub.1 and t.sub.2, the distance between the surface of the N type epitaxial layer 3 and the N.sup.+ b.sub.1 region and between the surface of the N type epitaxial layer 3 and the N.sup.+ b.sub.2 layer, respectively are then adjusted.
Next, as shown in FIG. 2c, P type impurities are simultaneously diffused under the same conditions in the desired regions of both element regions 1 and 2 on the N type epitaxial layer 3 so as to form P type regions 6, 7, 8 and 9, all having the same required depth, within the N type epitaxial layer 3. The distance d.sub.1 between the P type regions 6, 7, 8, and the buried regions N.sup.+ b.sub.1 and the distance d.sub.2 between P type region 9 and buried region N.sup.+ b.sub.2 are then adjusted.
However, the above conventional process requires two photo processing steps and two diffusion processing steps to form N.sup.+ type diffusion regions N.sup.+.sub.AS and N.sup.+.sub.P for the formation of buried regions, so that the processes are complex. Further it suffers from the disadvantage that the phosphorus which is used in forming the buried region has a large autodoping effect to the N epitaxial layer 3. The concentration of phosphorus in the N.sup.+ diffusion region N.sup.+.sub.P must be limited to keep the amount of autodoping below the tolerance, thus there is the problem that one cannot obtain the resistance value of the buried region N.sup.+ b.sub.1 most suitable for the element characteristic. Further, there is the problem that the large diffusion coefficient of the phosphorus makes it very difficult to control the diffusion in the vertical direction.